The present invention relates generally to wireless communication systems and, in particular, to handoff in a spread-spectrum communication system.
Wireless communication systems are known to employ handoff methods in which a remote unit (such as a cellular subscriber) is handed off between base stations within the communication system. In particular, when a remote unit moves to the periphery of the serving base station""s coverage area, the increase in path losses between the serving base stations and the remote unit may create a situation in which an adjacent base station can better serve the remote unit. As described in the Electronic Industry Association/Telecommunications Industry Association Interim Standard 95A (TIA/EIA/IS-95A), one such communication system employing handoff is a Code-Division, Multiple-Access (CDMA) spread-spectrum communication system. (EIA/TIA can be contacted at 2001 Pennsylvania Ave. NW Washington D.C. 20006). Handoff in a CDMA communication system may be accomplished by simultaneously monitoring several base stations (referred to as an active set) and replacing a base station within the active set if a base station with a stronger signal is encountered. Although TIA/EIA/IS-95A provides for a method for handoff between a CDMA base station and a base station utilizing other communication system protocols (such as the Advanced Mobile Phone Service (AMPS) protocol), TIA/EIA/IS-95A does not provide when such handoff should occur. Because of this, it has been left up to equipment manufacturers to develop methods for handing off from CDMA to other system protocols.
A prior-art method for handoff from a CDMA to an AMPS protocol is illustrated in reference to FIG. 1. Referring to FIG. 1, communication system 100 includes a plurality of CDMA base stations 101-107 having respective coverage areas 122-128. Communication system 100 additionally includes a plurality of AMPS base stations 109-115 having respective coverage areas 126-132. Remote unit 117 is capable of operating on both an AMPS and a CDMA protocol. At time t=1, remote unit 117, traveling along path 119, is in soft handoff (i.e. communicating with more than one base station) with non-border CDMA base stations 101 and 103. (Non-border base stations are those CDMA base stations with no corresponding AMPS coverage area). At time t=2, remote unit 117 moves into coverage area 126 and continues in a soft-handoff state communicating with non-border CDMA base stations 101 and 103, and additionally communicating with border CDMA base station 105. At time t=3, remote unit 117 moves out of coverage area 124 and is placed in soft handoff with non-border CDMA base station 101 and border CDMA base station 105. Finally, at time t=4, remote unit travels out of coverage area 122 communicating only with border CDMA base station 105.
The prior-art method of handoff from a CDMA to an AMPS protocol immediately hands off remote unit 117 to AMPS base station 109 when remote unit 117 is no longer communicating with a non-border CDMA base station. In other words, remote unit 117 is handed off to AMPS base station 109 immediately upon losing communication with non-border CDMA base station 101. This prior-art method of handoff from a CDMA to an AMPS protocol is inefficient in that CDMA capacity may be wasted by prematurely handing off remote unit 117 to AMPS base station 109.
Thus a need exists for a method and apparatus for handoff in a spread-spectrum communication system that overcomes the deficiencies of the prior art.